1. Field of Endeavor
The present invention relates to a NOx sensor system and more particularly to a current-biased potentiometric (chronopotentiometric) NOx sensor system for vehicle emissions.
2. State of Technology
EP0 Patent No. 769,694 published Apr. 23, 1997 for NOx sensor and method of measuring NOx issued to NGK Insulators, LTD invented by Nobuhide Kato et al. provides the following background information, “Various measuring methods and devices have been proposed for determining the concentration of NOx in a measurement gas. A known method, for example, employs a sensor comprising a Pt electrode and an Rh electrode formed on an oxygen ion-conductive solid electrolyte such as zirconia . . . Disclosed is a NOx sensor and a method of measuring NOx capable of obtaining a large change in signal to measure a concentration of low concentration NOx in a measurement gas continuously and accurately with good response over a long period of time. A NOx sensor comprises a first internal space into which the measurement gas is introduced through a first diffusion rate-determining passage, a second internal space arranged with a NOx-reducing catalyst, into which an atmosphere is introduced through a second diffusion rate-determining passage, an electrochemical pumping cell for controlling a partial pressure of oxygen in the internal space by using a first oxygen ion-conductive solid electrolyte and electrochemical cells provided in contact therewith, a partial oxygen pressure-detecting means for detecting the partial pressure of oxygen in the internal space by using the first oxygen ion-conductive solid electrolyte and electro-chemical cells provided in contact therewith, a first electrochemical sensor cell for outputting an electromotive force corresponding to the partial pressure of oxygen in the internal space, and a voltage-detecting means for detecting the electromotive force outputted from the first electrochemical sensor cell. The NOx concentration is determined from a value of the electromotive force of the first electrochemical sensor cell detected by the voltage-detecting means.”